Double or multiple sheets of glass separated by a gas space exhibit a substantially lower thermal conductance than a single sheet of glass. The thickness of the gas space must be large enough to reduce the amount of heat conducted through the gas, but small enough to prevent convection currents from being set up. The gas space may be partially evacuated to inhibit convection, but very low pressures must be reached if conduction is to be inhibited.
There are several difficulties to address in the design and production of very low pressure evacuated windows: for instance, to achieve a very low level of vacuum and maintain it over a long period of time, it is necessary to make a seal around the perimeter of the window using materials which have extremely low permeability to gases and negligible outgassing over long periods of time. Normal gasket sealing materials such as rubbers and adhesives are not suitable.
In addition, an array of supports must be provided between the sheets of glass to ensure sufficient mechanical strength to withstand the forces imposed by atmospheric pressure. These supports lead to localized concentrations of mechanical stress in the glass, and in the supports themselves, which increases the danger of breakage resulting in hazardous implosions. Furthermore, the mechanical supports lead to an increase in thermal conduction through the window.
Glass itself contains dissolved gases which may be emitted into the evacuated space over long periods of time. It is therefore necessary for the structure of the window to be heated to high temperatures while it is being evacuated, to eliminate any gas dissolved within the glass. But, at high temperatures glass is significantly weaker than at room temperature. This limits the temperatures which may be used, and consequently increases the time required, to eliminate the gases dissolved in the glass.